1. Field of the Invention
The invention relates to methods and apparatus for forming polymer film, sheet or tube comprising a mechanically isotropic liquid crystal polymer (LCP) and a thermoplastic polymer (TP). It relates more particularly to forming a film which inherently maintains its shape and has a more uniform coefficient of thermal expansion than has been obtainable previously.
The invention also relates to methods and apparatus for forming a coextruded polymer structure comprising at least one LCP layer and at least one TP layer, at least the LCP layer having a controlled molecular orientation.
2. Description of Related Art
The invention relates in general to the formation of multiaxially oriented films from high-molecular-weight liquid crystalline thermotropic or lyotropic polymers (homopolymers, copolymers, and the like), under processing conditions whereby the films have a controlled molecular orientation. These polymers generally fall into two classes, those that are modified in solution form, i.e., lyotropic liquid crystalline polymers, and those that are modified by temperature changes, i.e., thermotropic liquid crystalline polymers. For a shorthand expression covering both types of polymers, the present disclosure will use the term "liquid crystal polymers" or LCPs.
In certain embodiments disclosed herein, a liquid crystal polymer is co-extruded with a thermoplastic polymer.
The liquid crystal polymers concerned herein are believed to have a fixed molecular shape, e.g. linear, or the like, due to the nature of the monomeric repeating units comprising the polymeric chain. Linear ordered polymers are also known as "rod-like" polymers. These liquid crystal polymers can be blended with the more common, typical "coil-like" (thermoplastic) polymers in which the molecular chain does not have a relatively fixed shape. Some of these blends have processing and functional characteristics similar to liquid crystal polymers, and to that extent, these blends are to be considered as being included in the invention disclosed herein.
Preferred LCPs are of the class of aromatic copolyesters, exemplified by commercial products such as Vectra.RTM. (Hoechst-Celanese), Xydar.RTM. (Amoco Performance Products), HX type LCPs (DuPont), Eikonol.RTM. and Sumikasuper.RTM. (Sumitomo Chemical), Rodrun.RTM. (Unitika) and Granlars (Granmont). Preferred "coil type" or thermoplastic polymers are thermoplastics such as polyethylene, polypropylene, ethylene vinyl alcohol, polyvinylidene fluoride, polyether ether ketone, polyarylate, polyamide, polyamide-imide, polycarbonate, polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, polyether imide, polyimide, polyphenylene sulfide, polystyrene, polyurethane, and polyvinylidene chloride. These are typical examples and do not restrict the invention.
Liquid crystal polymer films have desirable qualities in a number of applications, but significant drawbacks related to their mechanical anisotropy. They are useful in particular for forming high gas barrier films and circuit substrates. The gas barrier properties of LCPs are typically 100 to 300 times better than polyethylene terephthalate, making them useful for food and beverage packaging applications. Circuits can be formed on such a film by plating and etching, and then a plurality of such circuits can be laminated, to form a circuit board having multiple circuits accommodated within the board. Flexible circuits can also be formed on liquid crystal polymer films.
However, previously the mechanical properties of liquid crystal polymer films have been inadequate for these applications. They cannot be blown and drawn after extrusion as coil polymers can, since they become too highly oriented in the die. They are too weak in the non-orientation directions to be stretched after extrusion, even while in semi-flowable form.
To improve their strength, liquid crystal polymer films are typically extruded between a pair of concentric counter-rotating cylindrical dies to form a tube. This process causes the inner and outer surface layers of the tube to have different respective directions of orientation, and this gives the tube biaxial strength and permits blowing and drawing, if desired.
FIGS. 2A-2C are schematic representations of extruded films showing the morphology of the oriented polymer material layers therein. In FIG. 2A, with no transverse or circumferential shear, the film has a uniaxial orientation, with all molecules oriented in the machine direction, that is, longitudinally with respect to the direction of flow through the die. In FIG. 2B the film has a biaxial orientation. The molecules in the top portions of the film are oriented at an angle of +theta with respect to the machine direction while the portions of the film in the lower part of FIG. 2B are oriented at an angle of -theta to the machine direction. FIG. 2C shows a planar isotropic film wherein the polymer rods lie randomly in the film plane, not strongly oriented at any specific angle with respect to the machine direction.
A biaxially oriented tube can be slit and spread apart to form a flattened film structure. However, the present inventors have observed a problem with this process which was not previously understood. The films thus formed will not lie flat. Although such a film can be flattened by pressing under heat, it has been observed that the film regains its tendency to curl as it continues to cool after pressing. Simply described, the two surface layers of the film inherently have different coefficients of thermal expansion (CTE) axially and transversely to the orientation of their molecules. Generally, the transverse CTE is greater. So as the sheet cools, each layer will try to shrink more in its own transverse direction. But since the two layers are both part of the sheet, the sheet as a whole cannot freely shrink in either direction. As a result, stresses are stored in the layers, which makes the sheet bistable, whereby it is able to hold a curl about either of two different axes and readily adopts one of these two conditions unless an active effort is made to hold it flat.
As best understood, liquid crystal polymer films have this curling problem because they are fibrillar, i.e., they comprise relatively straight molecules. The molecules orient strongly in the die and the flowing polymer becomes anisotropic, more so than ordinary coil polymers which tend to randomize. A coil polymer tube or sheet can be strengthened biaxially throughout its entire thickness by blowing and drawing after it exits from the die. Sometimes counter-rotating dies are also used to make conventional polymers more isotropic. But the combination of shearing and stretching is much more critical and difficult to optimize with liquid crystal polymer extrudates, since they readily become highly oriented in the die anisotropically. It may not be possible to stretch the polymer substantially in the direction transverse to its fibrillar orientation.
If counter-rotating annular dies are used, to establish a biaxial or multiaxial (specifically, twisted nematic) orientation of the molecules in the flow, then transverse stretching by blowing of the extruded tube is possible and effective.
But, as mentioned above, such a process forms two layers in the film with complementary orientations, for example +/-45.degree., on either side of the machine direction in which the extrusion has taken place. As described above, this has led to the drawback of curling in liquid crystal polymer film sheets made from such extruded tubes. The liquid crystal polymer films become less anisotropic due to the application of transverse shear, but they still curl after cooling, because of the non-uniform CTE phenomenon mentioned above. Curl becomes very significant when the film is orthotropic, i.e., having equal properties in orthogonal directions in the plane of the film, as in a balanced biaxial film.
Another problem frequently associated with films produced by the tubular bubble process is seaming. Seams have been formed in some known methods in which film tubes are flattened or "blocked" as they are driven through pinch rolls.
These problems relate at least in part to inherent characteristics of the tubular extrusion process, and in part to the methods of system control and downstream processing, beginning with coagulation or cooling, and perhaps in part also to inadequate dope homogeneity upstream.
No techniques previously known to the art have been able to solve these problems.
U.S. Pat. No. 5,248,530 of Jester et al., issued Sep. 28, 1993 and assigned to the Hoechst-Celanese Corp., discloses coextrusion of a higher-melting LCP layer laminated with a lower-melting LCP layer. Jester et al. also discloses sandwiching the higher-melting LCP layer between two lower-melting LCP layers.
U.S. patent application Ser. No. 206,137, now U.S. Pat. No. 4,963,428, filed Jun. 13, 1988; Ser. No. 203,327 filed Jun. 7, 1988, now U.S. Pat. No. 4,939,235; and Ser. No. 098,710 filed Sep. 21, 1987, now U.S. Pat. No. 4,973,442; all commonly assigned herewith, disclose processes wherein biaxially oriented, substantially two-layer, liquid crystal polymer films are formed in counter-rotating annular dies by controlling the transverse shear speed, the material flow rate, the blow ratio and the draw ratio, all of which affect the molecular orientation in the final product, to obtain a substantially +/-45.degree. orientation of the two surface layers. See also U.S. Ser. No. 209,271 filed Jun. 20, 1988, now U.S. Pat. No. 5,135,783.
Nagasawa et al., Japanese Disclosure No. 53-47460, discloses a manufacturing method for a lyotropic liquid crystal polymer film which includes applying transverse shearing forces to the dope. See FIG. 2 and pp. 8-9.
Other prior art of interest includes:
Urasaki, Japanese Disclosure No. 53-86798 PA1 Sugimoto et al., Japanese Disclosure No. 54-44307 PA1 Fujii et al., Japanese Disclosure No. 63-199622 PA1 Fujii et al., Japanese Disclosure No. 63-173620 PA1 Inada et al., Japanese Disclosure No. 52-109578 PA1 Miyamoto et al., Japanese Disclosure No. 63-296920 PA1 Donald, U.S. Pat. No. 3,279,501 PA1 Donald, U.S. Pat. No. 3,404,203 PA1 Sharps, Jr., U.S. Pat. No. 4,496,413 PA1 Isayev et al., U.S. Pat. No. 4,728,698 PA1 Helminiak et al., U.S. Pat. No. 4,377,546
The respective disclosures of all the prior art materials mentioned herein are expressly incorporated by reference.